Perhaps the most popular battery chemistries that have hit the consumer market recently are lithium-based systems. Lithium batteries use high valence metal oxide materials, which are reduced during the electrochemical reaction. This reaction in rechargeable lithium and rechargeable lithium ion batteries must be fully reversible in order to have a commercially viable cell. These electrochemical systems include manganese-based lithium metal oxides configured in lithium, lithium ion, and lithium polymer electrochemical cells. Common reversible metal oxide materials used in lithium batteries include LixMn2O4, LixMnO2, LixCoO2, LixNiO2, and LixNiyCozO2.
Today, rechargeable lithium batteries are used in portable electronic devices including cellular phones and laptop computers. Future use of rechargeable lithium battery systems is targeted at applications related to electronic vehicles and pairing with fuel cells to produce high-energy systems with excellent pulse capabilities. Lithium batteries have the flexibility of being packaged into either cylindrical or prismatic cell designs; this feature makes them applicable to almost any portable electronic system where battery volume is a concern.
The benefits of lithium battery systems include high specific energy (Wh/kg) and high energy density (Wh/l). Lithium electrochemical systems produce a relatively high nominal voltage between 3.0 and 4.75 volts. Lithium electrochemical systems can operate between 3.0 and 4.35 volts or between 2.0 and 3.5 volts. Additionally, lithium electrochemical systems have excellent charge retention due to a low self-discharge rate.
Manganese dioxide (MnO2) based materials are attractive for use as a cathode material in lithium electrochemical systems. MnO2 is attractive because of its high energy density and low material cost. MnO2 is an active material which creates a skeletal structure that allows lithium cations to fill vacancies and voids within the structure. Ideally, this structure does not change with cycling; altering of this crystal structure may cause capacity fading. Additionally, the MnO2 active material exists in different forms. These forms include a lithiated spinel (LixMn2O4) and its different structures denoted by α, β, γ, and λ. In lithium electrochemical cells, the active material is bound to an aluminum current collector with either polytetrafluoroethylene (commercially available under the trade name TEFLON, manufactured by DuPont) or polyvinylidene fluoride mixed with conductive carbon. The conductive carbon serves as an aid for electron transfer.
Capacity fading is a major problem for rechargeable lithium cells. Capacity fading is the loss of cycle capacity in a cell over the life of an electrochemical system, limiting the practical number of cycles that may be used. In lithium battery systems, capacity fading is often attributed to the degradation of the active cathode material. This cyclic capacity loss is a result of both changes in composition and crystal structure of the active cathode material. Additionally, throughout the life of a cell, parasitic side reactions occur between chemical species of all cell components. Methods of reducing this effect include modifying the crystal structure and/or composition of the active material.
Capacity fading associated with the cathode material has also been linked to the fracture of active material and the dissociation or disconnection of the fractured active material from the electrode. Fractures are caused by mechanical stress-strain of MnO2 crystal structures during cycling of the cell. Stress-strain forces act on the crystal structures as a result of repeated phase transitions. These stress-strain forces are due to the insertion and extraction of lithium in the cathode lattice. This frequent conversion in geometry and dimension of the crystal lattice creates a significant mechanical strain on the cathode. This mechanical strain is believed to electrically disconnect active material from the electrode through fracture. Additionally, an external influence, such as elevated temperature, can also promote cathode fracture. In this case, structural vibrations increase with temperature, resulting in the disconnection of the fractured active material from the electrode.
Another major cause of capacity fading in manganese-based cathodes is the dissolution of manganese into the electrolyte. Through a series of chemical reactions, manganese (Mn2+) is removed from the cathode and dissolved into the electrolyte, resulting in a decrease of active material in the cathode. Manganese dissolution is linked to reactions with the electrolyte and, more importantly, the impurities dissolved within the electrolyte. Many of these reactions are linked to the water content of the electrolyte and the presence of hydrofluoric acid (HF). The products of parasitic reactions are phase transitions of the MnO2 structure, which results in the formation of Mn2O3 and Mn3O4.
Manganese dioxide (MnO2) provides a skeletal background for lithium intercalation during cycling of a lithium electrochemical cell. When fully charged, manganese particles have a meta-stable 4+ valence state. This meta-stable 4+ valence state allows for the attraction and intercalation of lithium cations into the lattice structure. As lithium cations fill the skeleton crystal structure during discharge, the crystal structure of the active material changes. Charging of the cell removes these lithium cations from the cathode, again altering the crystal structure. Ideally, this is a completely efficient and reversible process, but realistically, continuous crystal structure changes lead to phase transitions that can impede lithium mobility. As a result of these phase changes, unwanted crystal structures develop that are either too stable for electrochemical reactions or block the insertion/extraction paths of lithium cations into the cathode material. This general phenomenon is regarded as the major contributor to capacity fading.
MnO2 exists in several phases or crystal structures and are referred to by the following prefixes: α, β, γ, and λ. α-MnO2 is the most stable MnO2 structure. α-MnO2 is one-dimensional and the lattice contains both one by one and two by two channels for lithium insertion/extraction. β-MnO2 is a tetragonal structure with the lattice containing one by one channels for lithium insertion/extraction. γ-MnO2 is also one-dimensional, existing in both hexagonal and orthorhombic crystal structures with a lattice that contains one by two channels for lithium insertion/extraction. Because of their stability α-MnO2, β-MnO2, and γ-MnO2 are not considered rechargeable. However, cycling of lithium into the α-MnO2, β-MnO2, and γ-MnO2 lattice can be achieved with rigid stoichiometric control.
λ-MnO2 is the conventional form of MnO2 based cathode material for rechargeable lithium electrochemical systems. λ-MnO2 is created through the delithiation of LixMn2O4 AB2O4 spinel. The λ-MnO2 crystal structure is maintained through both charge and discharge of the LiMn2O4 spinel. The maintenance of the λ-MnO2 structure during insertion and extraction of lithium in the LixMn2O4 spinel makes it an attractive couple with lithium for rechargeable electrochemical systems. The λ-MnO2 crystal structure is a three dimensional cubic array. This crystal structure promotes mechanical stability and adequate pathways for lithium insertion/extraction. Degradation of the λ-MnO2 crystal structure, forming α, β, or γ-MnO2 crystals and other MnxOy phases, reduces the capacity of the cathode material.
As lithium intercalates, the size and orientation of the crystal structures change. In LixMn2O4 spinel materials, when 0.05<x<1, the crystal structure is cubic (λ-MnO2). When 1<x<1.8, the structure of LixMn2O4 (no longer a AB2O4 spinel) is tetragonal. Additionally, when x<0.05, a phase transition to the more stable α, β, and γ MnO2 can occur. Continued charge and discharge promotes the transformation of the cubic crystal structure to other cubic, tetragonal, and monoclinic phases. Tetragonal and monoclinic crystal structures may become inactive, leading to the loss of active cathode material.
Voltage control, maintaining 0.05<x<1, allows for the mitigation of the formation of unwanted crystal structures. When the potential of the lithium/LixMn2O4 electrochemical system is maintained between 3.0 and 4.25 volts, the cubic phase is maintained. Once the potential of the system drops below 3.0 volts, the LixMn2O4 cathode material undergoes a phase change from cubic to tetragonal. When the potential of the system increases above 4.25 volts, the LixMn2O4 cathode material becomes stripped of the lithium component and undergoes a phase change from cubic (λ-MnO2) to the more stable α, β and/or γ MnO2.
Other phase transitions that lead to capacity fading include the formation of Mn2O3 and Mn3O4. The Mn2O3 and Mn3O4 formations result from the liberation of oxygen in the MnO2 and Mn2O4 structures. The valence state of manganese in these structures is 3+ or less. This lower valence state creates a stable crystal structure that is not conducive to lithium intercalation and, therefore, not rechargeable. As more Mn2O3 and Mn3O4 are formed, less MnO2 and Mn2O4 remain and the usefulness of the cathode decreases.
Thus, one of the disadvantages of conventional lithium manganese-based AB2O4 spinel materials is the limited cycle life and limited rate capability for lithium electrochemical systems. Furthermore, this problem is a major obstacle for rechargeable lithium battery technology. An additional limiting factor for lithium manganese-based AB2O4 spinel materials is the time required to process the raw materials and synthesize the desired product; conventional methods require multiple mixing, grinding and calcining steps, which take days to complete.